The performance of electrodes modified with electrochemically generated cobalt oxide nanoparticles for the oxidation of As(III) species was investigated by cyclic voltammetry (CV) and rotating disk voltammetry (RDV). The oxide nanoparticles were made by electrodeposition from cobalt oxides using CV without the reduction of water or anions. Controlling deposition parameters, different values of surface concentration (Γ) can be obtained. Electrochemical experiments (CV and RDV at different rotation rate) showed a shift in the As(III) oxidation potentials to lower values; when the coverage surface increased, these differences in the surface concentration produced a potential shift of up to 150 mV. This phenomenon depends on the ratio of the electrode active area to the geometric area (Ψ). The Levich and Koutecký-Levich analysis of RDV voltammetric data confirmed that the oxidation of As(III) on modified electrodes is controlled by mass transport. It was also demonstrated that different values of surface concentration produces different kinetic current values.